Pigment Yellow 138 is well known to be a very lightfast and weatherfast green-shade yellow pigment with good heat stability. Pigment Yellow 138, also known as quinophthalone, is typically used in plastics and paint applications. The most common commercial Pigment Yellow 138 has a relatively low specific surface area of approximately 25 m2/g, which gives good hiding power in systems where opacity is desirable. For most ink systems however, it is desirable to have a high degree of transparency, therefore, pigments having a high specific surface area are required.
Pigment Yellow 138 may be prepared by various means of comminution, including ball milling or kneading in the presence of a grinding agent. However, it has not been know how to prepare Pigment Yellow 138 having a high surface area exhibiting strong color strength and transparency while also having lightfastness.
The preparation of quinophthalone pigments is described in the prior art. For example, condensation of 2-methylquinoline derivatives (quinaldine derivatives) with aromatic ortho-dicarboxylic acids or their anhydrides is carried out in the presence of inert high-boiling diluents such as ortho-dichlorobenzene, trichlorobenzene, nitrobenzene, naphthalene, diphenyl or diphenyl ether (Chimia 24, 328, 1970; GB-A1,091,734). U.S. Pat. No. 5,106,980 discloses a process for the preparation of quinophthalones by condensation of 8-aminoquinaldine with phthalic anhydride optionally monosubstituted or polysubstituted by chlorine or bromine, in the presence of a diluent, resulting in relatively low specific surface area and opaque pigment. U.S. Pat. No. 3,622,583 describes a process for the preparation of yellow dye quinophthalone derivatives by heating 8-aminoquinaldine with tetrachlorophthalic anhydride or tetrabromophthalic anhydride in the presence of other solvents and at a temperature of 130–300° C. According to the Industrial Organic Pigments, by Herbst, W. and Hunger, K. VCH., Yellow 138 pigments that possess higher surface area exhibit somewhat lower lightfastness properties. However, the prior art processes result in relatively low specific surface area Pigment Yellow 138 and therefore are mostly opaque.
Thus, there exists a need for Pigment Yellow 138 having increased transparency and lightfastness while maintaining standard coloristic properties in a printing ink such as color strength and tint strength.